The present invention relates to a solid type sensor for a nitrogen oxide gas, and more specifically to a nitrogen oxide gas sensor suitable for measuring NOx discharged from general combustion system and NOx in indoor environments, and particularly for sensing NOx in the exhaust gas of automobiles, where the gas temperature may reach quite high degree.
A gas sensor of the system of forming a sensing electrode and a counter electrode thereof on a zirconia solid electrolyte substrate and sensing the concentration of NOx by measuring the potential difference between the electrodes has already been reported. For example, gas sensors described in Japanese Patent Laid-Open Publication No. Hei 7-198671 and Japanese Patent Laid-Open Publication No. Hei 8-43346 each has a construction that a sensing electrode made up of a metal oxide such as CdMn2O4 or NiCr2O4 and a Pt counter electrode are formed on a zirconia solid electrolyte substrate as an oxygen ion conductor and it can be said, therefore, that these gas sensors have a heat resistance capable of being used in a high-temperature atmosphere.
On the other hand, as a sensing electrode having a sufficient heat resistance in a high-temperature atmosphere such as in an exhaust gas of automobiles, etc., even a noble metal electrode can be expected to be used. In this point, a Pt electrode has been already used as an electrode in a xcex oxygen sensor of automobiles and a linear air-fuel oxygen sensor and the high reliance in the practically used has been already proved. The noble metal electrode has many merits of a chemical stability thereof, the easiness of the preparation thereof, capability of expecting the reduction of the impedance of the electrode, etc. The examples of the sensor relating to a NOx gas using a noble metal sensing electrode on a zirconia solid electrolyte substrate cited in Japanese Patent Laid-Open Publication No. Hei 8-271476 are as follows.
One of them is disclosed in U.S. Pat. No. 4,199,425, which illustrates a sensor prepared by forming an alumina overcoat layer impregnated with Rh for imparting a NOx sensing to a concentration-cell type oxygen sensor (xcex sensor) for automobiles. However, in this sensor structure, the role of the overcoat layer impregnated with Rh is a NOx decomposition catalyst layer and it is clear that oxygen itself formed by the decomposition of NOx is sensed by the Pt sensing electrode.
Another one is shown in Japanese Patent Laid-Open Publication No. Sho 59-91358, which discloses a sensor prepared by forming an electrode made up of a noble metal such as Pt, Rh, Pd, Au, etc., and a sensing electrode prepared by laminating or applying an N2O decomposition catalyst such as Co3O4 on the above-described electrode on a zirconia solid electrolyte substrate, and in which the potential difference between the electrodes is measured. However, in the case of measuring NOx in the exhaust gas from automobile engines, the target gases are NO and NO2 and thus by the sensor for N2O, the sum of NO and NO2 gases cannot be measured. Furthermore, the potential difference as sensor outputs to a low-concentration gas is very small and in the middle concentration range (several 1000 ppm or lower) of the real exhaust gas, the potential difference is almost same as zero.
As described above, although a noble metal sensing electrode is used in a concentration-cell type NOx sensor, the role thereof is simply as a NOx decomposition catalyst or only functions as a current collector of collecting electron charges generated in the decomposition reaction by the catalyst layer. Furthermore, as mentioned in Japanese Patent Laid-Open Publication No. Hei 8-271476, it is the present state that in the NOx sensor using a conventional noble metal sensing electrode, the potential difference as sensor outputs is small, the dependence on the oxygen concentration in a detection gas atmosphere is strong, and further the sensor can only be operated and sensing with decomposing NOx.
As described above, in the potential difference-type NOx sensor using an oxide electrode, a high sensitivity is obtained but the resistance of the sensing electrode is high and thus a current collector must be formed in the sensing electrode to reduce the electrode area.
On the other hand, in the noble metal electrode which is a good electric conductor as an electrode material, there is no such an electrode which measures the NOx potential difference as it is. The noble metal electrode can have a little sensitivity only to N2O (laughing gas). Furthermore, in the case of noble metal electrode, it is necessary to control the O2 concentration correctly because the potential difference depends on oxygen partial pressure.
In view of these problems, an object of the present invention is to provide a potential difference-type NOx sensor having a low sensor impedance, using a noble metal electrode having a good electrode conductivity, and having excellent NOx sensing characteristics. Furthermore, the object of this invention is also to provide a potential difference-type NOx sensor capable of measuring the NOx concentration without being influenced by the oxygen partial pressure of an atmosphere even in the case of applying it to exhaust gases sensing, etc., from automobiles.
The present inventors have found that the above-described object can be attained by the present invention as set forth hereinafter.
That is, the present invention provides (1) a nitrogen oxide gas sensor of a system of measuring a potential difference between a sensing electrode formed on a zirconia solid electrolyte substrate which is an oxygen ion conductor and a platinum counter electrode or a platinum reference electrode insensitive to NOx, which makes a pair with said sensing electrode, on the above-described solid electrolyte substrate, wherein an alloy electrode comprising platinum and rhodium or a cermet electrode comprising platinum, rhodium, and zirconia is used as the gas sensing electrode of said sensor, and (2) the nitrogen oxide gas sensor of (1) wherein in the alloy electrode comprising platinum and rhodium or the cermet electrode comprising platinum, rhodium, and zirconia, the sensing electrode containing rhodium at least 0.5% by weight to the sum total of platinum and rhodium is used.
Also, the present invention provides, as a total NOx sensor by combining the alloy electrode comprising platinum and rhodium or the cermet electrode comprising platinum, rhodium, and zirconia described above and a sensor structure, (3) a nitrogen oxide gas sensor of a system of carrying out a sensing by introducing gas to be sensed into a chamber formed inside the sensor body each composed of a zirconia solid electrolyte which is an oxygen ion conductor and having a structure that said chamber is composed of a 1st chamber having a gas inlet connected to a gas atmosphere to be sensed or a structure of said 1st chamber and a 2nd chamber connected to the 1st chamber, wherein said sensor comprising a pair of electrodes formed in the 1st chamber and the 2nd chamber for oxygen pumping-out or oxygen pumping-in, a means for controlling the oxygen concentration in the 1st chamber or the 2nd chamber, a sensing electrode for NOx converted into NO or NO2 in said 1st chamber, and a platinum counter electrode to the sensing electrode formed in the chamber where said sensing electrode placed or a platinum counter electrode formed such that it connects to a duct of keeping a standard oxygen concentration putting the zirconia solid electrolyte substrate between said sensing electrode and said counter electrode, and said sensing electrode is composed of the alloy electrode comprising platinum and rhodium or a cermet electrode comprising platinum, rhodium, and zirconia described above, and (4) the nitrogen oxide gas sensor of (3) wherein the sensor has a system that the oxygen concentration in the chamber having formed therein said sensing electrode is controlled such that the NOx potential difference is generated from the mixed potential to oxygen and NOx in the sensing electrode. By the nitrogen oxide gas sensor, the oxygen partial pressure dependence, which becomes a noise of the NOx sensing, can be substantially removed.
Explaining in more detail, platinum and rhodium each is used as an NOx catalyst but the alloy of them has never been used as a potential difference sensing electrode (active to oxygen and NOx) itself. Also, the assertion of the present invention is that the electrode of this invention is used by a different principle from a conventional concentration cell type. That is, the mixed potential [the electrode potential (potential difference to the counter electrode) determined by reactions of NOx and O2 detection on the sensing electrode] determined by simultaneously taking part in NOx and oxygen as the oxidation-reduction reaction of NOx (NO, NO2), is used as an output as the reaction of the NOx sensing electrode. As the sensor construction, if a sensing electrode 2 and a counter electrode 3 (inactive to NOx) are on a same zirconia solid electrolyte substrate as shown in FIG. 1 and FIG. 2, there is no restriction on the arrangement of them. In this case, however, oxygen must exist in the sensing electrode atmosphere to generate mixed potential. The counter electrode 3 must be inactive to NOx under the using condition and thus is usually formed of platinum only or formed of platinum added with zirconia for the control of the electrode microstructure.
As a matter of course, in the construction shown in FIG. 3, the reference atmosphere at the counter electrode 3 side may be fixed to the air.
Also, when NOx does not exist in the counter electrode 3 side in the construction shown in FIG. 3, it is clearly in the category of the present invention that the Ptxe2x80x94Rh alloy electrode, etc., for example, of this invention having a sensitivity to NOx can be used.
In FIG. 1, FIG. 2, and FIGS. 3, 4a and 4b show lead wires from the electrodes 2 and 3 respectively, and 5 shows an isolation walls for isolating the counter electrode 3 from a gas to be sensed.
Under such a condition, in a conventionally reported oxide electrode such as NiCr2O4, etc., the electric conductivity of the electrode film itself is low and thus it is necessary to form a current collector for catching reaction charges under the electrode. Because the electrode impedance of the oxide electrode itself is large, for example, when it is used for automobile, a noise is liable to occur and it is difficult to ensure the accuracy. Thus, even when it is intended to enlarge the size of the electrode, because of the low electric conductivity of the electrode itself, there occurs a problem that the potential difference can not be measured with good sensitivity without using a current collector.
On the other hand, although a noble metal electrode has a good electric conductivity of electrode, the electrode capable of sensing NOx as a mixed potential has not yet been found. Hitherto, noble metals used for a NOx sensor of a potential difference system is from the catalytic property or used as a simple current collector as described above.
The present invention is based on the consideration that by using an alloy film of Pt and Rh for a NOx sensing electrode, the oxygen adsorption of Pt and the catalysis of Rh are maintained on a same electrode, and the NOx potential difference by the above-described mixed potential is measured. Accordingly, the dispersion state of Rh (Rh concentration) and the sensitivity of NOx shall have a co-relation and in fact, such a result has been obtained.
However, the noble metal electrode is active to oxygen itself and when the sensing of a concentration cell type is carried out in, for example, the structure as shown in FIG. 3, the oxygen concentration fluctuation at the sensing electrode 2 side is directly sensed, whereby a precise control of the oxygen partial pressure becomes necessary in the sensing electrode atmosphere. In the region that an oxygen concentration is almost zero, practically, the measurement is carried out by only an oxygen concentration sensor but in this oxygen concentration region, the output dependence on the oxygen concentration is very strong and the precise concentration control is substantially impossible.
On the other hand, in the mixed potential-type sensor which is the application system of the present invention, the oxygen concentration dependence is very weak and even when the oxygen concentration control is substantially very rough, the NOx output is scarcely influenced. Thus, even in the circumstance which is hitherto considered to be used for automobile, the Ptxe2x80x94Rh alloy electrode of the present system can be practically applied.
FIG. 7 and FIG. 8 show sensor structures capable of sensing NO or NO2 in exhaust gases of automobiles as total NOx. In a 1st chamber, NO and NO2 in the exhaust gas is simplified into one of NO and NO2 by an oxygen pumping electrode disposed in the same chamber and the potential difference measurement is carried out in the 2nd chamber by the electrode of this invention. That is, in the case of sensing NOx as NO2, oxygen supply is carried out in the 1st chamber by the pumping electrode and the oxidation of NO is carried out. Conversely, in the case of detecting as NO by the reduction of NO2, the working voltage of the pump is reversed and oxygen is discharged.
In any cases, the oxygen concentration in the 1st chamber is feedback-controlled by the oxygen sensor disposed in the 2nd chamber connected to the 1st chamber. By incorporating the above-described mixed potential sensing system into the sensor structure shown in FIG. 7 or FIG. 8, the oxygen partial pressure dependence of the conventional noble metal electrode itself is largely moderated and the electrode can be applied to a sensor capable of sensing total NOx as NOx sensor for automobile.
Moreover, the present invention provides a nitrogen oxide gas sensor of a system of measuring the potential difference between a sensor electrode formed on a zirconia solid electrolyte substrate which is an oxygen ion conductor and a Pt reference electrode without having activity to NOx on the above-described solid electrolyte substrate, making a pair with said sensing electrode, wherein the gas sensing electrode of said sensor is an electrode formed by laminating a rhodium layer on a platinum layer or a cermet electrode formed by dispersing zirconia in the above-described laminated electrode, and the oxygen concentration of the measuring atmosphere is controlled to a definite value having an optional width of from 0.05 to 21% by volume.
Considering from the circumstance of being used in general rooms to the circumstance of being used in exhaust gases from motorcars, the effects of the present invention are as follows.
In the system of measuring the NOx concentration by the potential difference, by using the alloy electrode of Ptxe2x80x94Rh, the laminated electrode of Pt and Rh, or the cermet electrode of Pt, Rh, and zirconia each being the electrode in this invention, a very large sensor output, which is never been obtained by conventional noble metal electrodes, is obtained. Thereby, the measurement accuracy of the NOx concentration is greatly improved.
By using the alloy electrode of Ptxe2x80x94Rh, the laminated electrode of Pt and Rh, or the cermet electrode of Pt, Rh, and zirconia, the electric conductivity of the electrode itself is improved and it becomes unnecessary to form a current collector on a sensing electrode.
In the method of carrying out sintering the electrode with a zirconia green sheet in a body, the problems of the evaporation of the electrode material observed in conventional oxide electrode materials and inferior adhesion do not occur.
Also, by disposing the electrode of this invention in the chamber having an oxygen concentration controlled to a certain extent, the oxygen partial pressure dependence of the electrode itself is removed and the measurement accuracy in practical sensor driving is greatly improved.